THE PROBLEM OF THE ANORTHOSITES 225 



necessary that syenite and anorthosite should always grade imper- 

 ceptibly the one into the other. The Adirondack area is one of 

 considerable disturbance. It no doubt suffered some disturbance 

 at the time of the intrusion of these igneous masses, and it has 

 unquestionably been much faulted since their consolidation. Is it 

 reasonable to suppose that the region necessarily stood stock-still 

 during the long period required for the consolidation of these 

 igneous masses? It is, in fact, likely that faulting took place 

 during this period as well, and if it occurred at a time when the 

 anorthosite was completely crystallized but the syenite still molten 

 then it is quite possible that syenite might thus be brought laterally 

 against, and acquire an intrusive relation to, anorthosite. The fact 

 that syenite invades anorthosite locally need not therefore be fatal 

 to the conception of gravitative differentiation of these two types, 

 nor does it necessarily indicate the order of their arrival from the 

 depths. A diagrammatic simplicity is not to be expected, but the 

 broader relations, including the substantial freedom of the whole 

 interior of the anorthosite area from protrusions of syenite, seem 

 to give a distinct preference to their arrangement substantially as 

 layers with the syenite above as outlined in the foregoing. 1 



The writer's leaning toward differentiation practically in place 

 as the explanation of the variation of many batholiths has been 

 criticized publicly by Harker, 2 and by others in private corre- 

 spondence. It is apparently believed that when one rock invades 

 another the relation necessarily means that the invading rock 

 arrived from a deep-seated magma basin subsequent to the other. 

 This may be quite true, as a rule, but there is little evidence in most 

 cases that adequate consideration has been given to the alternative 

 view of differentiation practically in place with only relatively 

 minor disturbance during the magmatic period. The petrologist 

 should be reluctant to reject this possibility without fair trial, for 

 he destroys some of the hope of solving the problems of igneous 



1 A few small patches of syenite have been found within the anorthosite area. If 

 these are regarded as having been pushed up from below as pipes, it is rather remark- 

 able that in no instance has their intrusive nature been demonstrated. On the other 

 hand, if they are remnants of an overlying syenite, they might well lack a definite 

 intrusive character. 



2 Jour. GeoL, XXIV (1916), 554. 



